The present invention relates to a measuring device and a method for determining the position of an electrically conductive test object with a noncontacting sensor, in particular an eddy current sensor, wherein the test object is adapted for linear reciprocal movement in a predeterminable direction.
Measuring devices and methods of the type under discussion have been known from practical operation in a great variety of designs and configurations, for example, from DE 101 41 764 A1; U.S. Pat. No. 6,762,922; and U.S. Patent Publication No. 2003/0098686. Quite generally, such measuring devices and methods are used to define distances, displacements, or positions relative to any electrically conductive test objects. Typical applications of noncontacting displacement measuring sensors, in particular eddy current sensors, include, for example, the positioning of wafer slices in the semiconductor production, the detection of vibrations or bearing oscillations, or the monitoring of air gaps in magnetic bearings. A specific application of noncontacting displacement sensors, to which the invention relates in particular, is the determination of test objects that linearly reciprocate in one direction, for example, the measurement of immersion depths, or the monitoring of piston strokes or cylinder positions.
In the case of the measuring devices of the prior art, the sensor is generally arranged in the extension of the longitudinal axis of the component being detected, and is aligned parallel to same. Because of the linear reciprocal movement of the component, for example, a cylinder piston, the distance between the end face of the cylinder piston and the sensor increases or decreases in accordance with the actual position of the cylinder piston. When an eddy current sensor is used, differently strong eddy currents are induced in the piston as a function of the distance between the piston end face and the sensor, which results in a corresponding output signal on the eddy current sensor. The output signal varies linearly with the position of the piston. In the case of such arrangements, the relatively long structural forms are often problematic, which applies in particular when only a limited space is available for the measuring device.
Such a situation exists in particular in the case of fuel injectors, as are used, for example, in pump-nozzle injection systems or common-rail injection systems. To monitor and regulate the injection, it has been common practice to measure the movement of the injector needle as described above, toward the closing piston. Disadvantageous in this instance is, on the one hand, the long structural form of the fuel injector, which is not needed for the injection itself. This long structural form is mechanically extremely costly and with that cost intensive both in its manufacture and in its maintenance and care. Furthermore, it is disadvantageous that the position measuring of the injector needle is extremely subjected to tolerances, since the usable measuring point is very small, and since it is extremely difficult and possible only with great adjustment efforts to position the sensor in an exact manner.
It is therefore an object of the present invention to provide a measuring device and a method for determining the position of an electrically conductive test object of the initially described type, which make it possible to determine in a very precise manner the position of a test object with simple constructional means and in a compact structural form.